The present invention relates to data storage systems, and more particularly, this invention relates to methods and systems for avoiding latency associated with communications between a host system and a storage system.
In systems in which a host system communicates with one or more storage systems over Fibre Channel (FC) networks, Fibre Channel Protocol (FCP) operations may be used as a transport protocol between the systems. In addition, some systems may use a higher level protocol referred to herein as Fibre Channel extensions (FCX) which may include IBM zSeries High Performance Fibre Connectivity (FICON), also referred to as zHPF, e.g., ANSI FC-SB-3 Single-Byte Command Code Sets-3 Mapping Protocol for FC protocol; National Committee for Information Technology Standards (NCITS) T11/FC-SB-4 Transport Mode operation, etc. For FCX and/or FCP operations that do not require a XFER_READY message or the like to obtain a first bit of data from the host system, it is possible for a control unit to detect a busy or check condition. Before sending this status to the channel, the control unit must wait until it receives the data from the channel, which will transfer a sequence initiative to the control unit. After sending the busy or check condition status, the control unit is then required to perform several more operations: 1) send a subsequent status to the channel (on a new different connection/exchange) indicating it is now capable of handling new write operations, 2) complete that operation, and 3) wait for the channel to redrive the command. This can be quite long, e.g., several milliseconds.